Autumn is a great time to plant trees, shrubs and hardy winter plants, as the soil is neither too dry nor too wet. So when the sun is shining for a couple of days straight, it pays to get going in the garden.

If you leave your gardening until winter, walking on and digging in soil that's too wet can damage the soil structure, causing compaction and making it difficult for plant roots to penetrate. Interestingly, the air spaces are almost as important as the soil itself. When gardening, try to avoid trampling on the soil. Carry a board with you to walk on instead, or put small pavers in opportune spots in garden beds to perch on as you do the weeding. Likewise, if the soil is more like mud, don't bother digging until it has dried out to a damp, crumbly consistency.

There are plenty of materials we can use to improve soil structure. Some require digging in, others can be left to integrate slowly into the soil. Soils rich in organic matter are better aerated and healthier, so incorporate organic matter by digging in compost when the soil is nice and friable. Heavy soil can be improved through the addition of sand and gypsum, too.

Alternatively, you can apply mulch regularly. Shredded tree mulch for border gardens and trees, or pea straw for veges and annuals, will do the trick over time.

Autumn is nature's time to mulch. Deciduous leaves drop, protecting the soil and feeding the creatures that live in it. Organisms present in the soil, such as worms, springtails and microscopic fungi and bacteria, work together to break down and digest organic matter, gradually incorporating it into the top loamy layer of soil, making carbon and nutrients available to plants.

The balance between bacteria and fungi in the soil shifts depending on which plants are growing in it. Trees and perennials in forests and orchards have a beneficial relationship with soil fungi. To encourage this activity, mulch the soil surface with shredded tree mulch and dry leaves. Veges, annuals and grasses prefer bacterial systems in the soil. To encourage bacteria, dig compost, manure and green crops directly into the soil.

When we add organic fertilisers such as rotted animal manure, lime, dolomite and seaweed, we're helping to feed the soil and boost the numbers of these beneficial microbes. Healthy soil, teeming with life, is a perfect medium for growing plants.

How is soil created?

An easily observable example of the early stages of soil formation can be found on Rangitoto Island. The volcano has been active on and off for 1000 years, the last eruption being 550 years ago. Once the volcanic action ceased and the lava cooled for the last time, wind-borne seeds blew in from the mainland or were deposited in bird droppings. Hardy pioneer plants such as pohutukawa got a toehold in cracks and crevices of rock.

Over time, the leaves falling from these plants filled the crevices, mixing with rock dust formed by the weathering action of rain on the volcanic rock. Added to this mix were bird droppings, insect bodies and other organic matter and, in time, the combination of these ingredients created enough soil for more plants to establish.

There's already an interesting array of around 200 species of trees and plants growing on the island, and this soil-building process will continue until a coastal forest eventually forms.

Love it or leave it

I know a fascinating professor who has spent a lot of time visiting villages in remote rural China for his research on ancient agricultural systems. In those places, villagers are still practising soil and water management techniques developed thousands of years ago.

He stated that the villagers do not "grow plants" - instead, they "grow soil" that is exceptionally deep and very productive.

Apparently the only thing that causes strife between neighbours is when a cow gets out and walks through next door's garden, damaging the soil. To avoid this fate, cows are taken for walks on leads and kept well away from productive land.

Bear this in mind next time you find yourself trampling around in your garden on wet soil, or letting your cows or horses wallow in a wet, puggy paddock near a stream.

All that valuable soil will be compacted and unhealthy, or end up in the waterways, spoiling rivers and polluting harbours.

Soil takes such a long time to form and is our greatest resource, so we should look after it.

Build your soil

Is your vege plot or border garden looking a bit tired, compacted, or lacking fertility? Or would you like to hibernate over winter and avoid garden work? Try sowing a mixed green cover crop into freshly tilled soil.

• A green cover crop will protect the soil structure from winter rain because the plant roots will bind the soil together, preventing soil erosion and the leaching of valuable nutrients.

• After a green cover crop has grown, the soil will also be a little drier when spring comes around. This ensures an earlier return to soil cultivation and gardening.

• Chop up the green crop before it sets seed, and dig in at least three weeks before planting your spring garden.